The present invention relates to locating underground cables, pipes and utilities, and more particularly to a trace wire that is used to carry a locating tone which is detected by a hand-held receiver thereby pinpointing the precise location of the underground utility.
Buried underground, particularly in the modem urban environment, is a virtual maze of utilities. These include telephone, electricity, gas, cable television, fibre optics, traffic signals, street lighting circuits, drainage and flood control facilities, water mains, and waste water pipes. Frequently, they are buried in close proximity to one another and are susceptible to damage due to construction equipment excavating in their vicinity.
The clear identification and location of the underground utilities is of the utmost importance to avoid dig-ups and damage. However, the records are often poor with inaccurate utility locations and depths. Some lines are not even recorded.
A trace wire is often placed in a duct or buried directly along with the utility in an attempt to provide a means for locating the utility prior to excavation in the area. The utility is located by transmitting a locating tone on the trace wire. A special receiver with magnetic field detecting coils is used to sense the tone current travelling along the trace wire. By this means the path and depth of the trace wire, and therefore the utility may be determined.
Typical trace wires used range from commercially available PVC insulated building wire to marker tapes with integral tracing wire. In practice, the trace wires used are often less than adequate. The standard commercial grade wire is not well suited for this application. The smaller gage wires often break or are damaged and present a high attenuation to the tone signal, which limits the useful locate distance. Larger gauge conductors, such as an insulated #6 AWG, have been used to lower the attenuation rate in an effort to reach greater distances. This improvement is offset by an increase in size and weight, which degrades the blow-in and pull-in performance in duct installations. In particular, good blow-in performance requires a light weight and good rigidity to prevent buckling during the installation.
The present invention makes use of a unique combination of insulation materials and conductors to achieve optimum results for the trace wire application.
According to the present invention, there is provided a trace wire to be placed alongside an underground utility for long distance transmission of locating tones, said trace wire comprising:
a conductive core;
an electrically insulating sheath on the core, said sheath comprising:
an inner layer comprising a solid foam of dielectric material in contact with and symmetrically distributed about the core;
an outer layer of solid dielectric material surrounding the inner layer.
The trace wire may be used with underground utilities of any type, for example cables, pipes and ducts.
The use of this combination of a core conductor with insulating materials in a multi-layered design provides significantly improved properties over current commercially available insulated conductors when used as trace wires.
In the presently preferred embodiment of the invention, the trace wire construction includes a. 1.6 mm (14 AWG) hard drawn copper conductor to provide low resistance along with high break strength. The inner layer of the sheath is gas injected foamed polyethylene (PE) insulation applied to an overall diameter of 7 mm. The outer layer is solid, medium or high density PE applied over the first layer to an over all outer diameter of 8.5 mm. This dual-layer insulation exhibits an effective relative dielectric constant of about 1.6. The attenuation constant at 500 Hz is 0.227 dB per km maximum. The break strength is about 135 kg and the trace wire is light weight at about 60 kg per km.